The principate of the Roman empire had no use for the republican army with its intense loyalties to competing generals. Beginning with the first emperor, Octavius Caesar, the princeps totally replaced the citizen army with an apparatus of professionals dedicated to carrying out the emperor's will in peace, disaster or war. The emperor commanded this apparatus himself, assisted and advised by a quasi-secret intelligence service. This article describes the Military establishment of the Roman Empire maintained by the emperors on behalf of the Roman state.

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Augustus created a standing army, made up of 28 legions, each one consisting of roughly 6000 men. Additional to these forces there was a similar number of auxiliary troops. Augustus also reformed the length of time a soldier served, increasing it from six to twenty years (16 years full service, 4 years on lighter duties).

The standard of a legion, the aquila (eagle) was the very symbol of the unit's honour. The aquilifer was the man who carried the standard, he was almost as high in rank as a centurion. It was this elevated and honourable position which also made him the soldiers' treasurer in charge of the pay chest.

A legion on the march relied completely on its own resources for weeks. In addition to his weapons and armour, each man carried a marching pack that included a cooking pot, some rations, clothes and any personal possessions. Furthermore, to make camp each night every man carried tools for digging as well as two stakes for a palisade. Weighed down by such burdens it is little wonder that the soldiers were nicknamed 'Marius' Mules'.

The core of legionaries was composed of Roman citizens who served for about twenty years, more or less. The term varied with the times. Fame was not permitted to military professionals. Augustus tried to eliminate the loyalty of legions to their commanders, requiring an oath of allegiance directly to him.

All emperors, good or bad, knew the importance to their stability of retaining the loyalty of the legions. Good emperors generally experienced no command problems. In the case of bad emperors, the legions were apt to mutiny and take power into their own hands. Sometimes they would provide candidates for emperor. At other times their support was for sale.

Legionaries continued to move farther and farther to the outskirts of society, especially in the later phases of the empire.

There was a general realization by the principate that the empire was too extensive to be governed effectively from Rome. Efforts were made to limit expansion. Existing territory was walled in. Border states were kept in line by a combination of the stick and the carrot. Sudden punitive raids might devastate a tribal region, removing the tribe's ability to wage war. Or, a cooperative state might win the equivalent to our modern most favored trading partner.

A favored device for keeping neighboring states under control was to use their forces in the army against non-cooperative states. Eventually the empire was nearly entirely guarded by provincial troops. They did not feel loyalty to the emperor, but backed their own commanders. This system led in the 2nd and 3rd Centuries to a large number of military usurpers and civil wars. By the time of the military officer emperors that characterized the period following the Crisis of the Third Century the Roman army was just as likely to be attacking itself as an outside invader.

Both the pre- and post-Marian armies were greatly assisted by auxiliary troops. A typical Roman legion was accompanied by a matching auxiliary legion. In the pre-Marian army auxiliaries were Italians, and often Latins, from cities near Rome. The post-Marian army incorporated these Italian soldiers into its standard legions (as all Italians were Roman citizens after the Social War). Auxiliaries were then made up of foreigners from provinces distant to Rome, who gained Roman citizenship after completing their term of service. This system of foreign auxiliaries allowed the post-Marian army to strengthen traditional weak points of the Roman system, such as light missile troops and cavalry, with foreign specialists, especially as the richer classes took less and less part of military affairs and the Roman army lost much of its domestic cavalry.

At the beginning of the Imperial period the number of legions was 60, which Augustus more than halved to 28, numbering at approximately 160,000 men. As more territory was conquered throughout the Imperial period, this fluctuated into the mid-thirties. At the same time, at the beginning of the Imperial period the foreign auxiliaries made up a rather small portion of the military, but continued to rise, so that by the end of the period of the Five Good Emperors they probably equalled the legionaries in number, giving a combined total of between 300,000 and 400,000 men in the Army.

There has been much debate regarding how much weight a legionary was required to carry. 30 kg (ca. 66 lb) is generally considered the upper limit for an infantryman in modern day armies. Estimates have been made which, including the entire equipment and the 16 day's worth of rations, bring the weight to over 41 kg (ca. 93 lb). This figure presumes the lightest possible weights for each item. The actual weight would have been even higher.

However, the reference may have been to a sixteen days ration of hard tack (buccellatum), which was usually used to supplement the daily wheat ration (frumentum). By using it as an iron ration, it might have sustained a soldier for about three days. The weight of the buccellatum is estimated to have been about 3 kg, which, given that the wheat rations would add more than 11 kg, means that without the corn, the soldier would have carried around 30 kg (66 lb) —or much the same weight as today's soldiers.

The rank and file of the legion was composed of three basic classes of foot soldier. The miletis was the common soldier, enlisted in hard labor. The discens was the mid-level rank and file member, training in a specialized field, but not yet a specialist. The immunes was the highest level of foot soldier. The necessity for a legion to undertake tasks such as bridge building or engineering siege machines required such specialists, whose skilled trades gave them privileged status, granting them higher pay and allowing them to be 'excused from regular duties'. Among them would be medical staff, surveyors, carpenters, veterinaries, hunters, armourers - even soothsayers and priests. When the legion was on the march, the chief duty of the surveyors would be to go ahead of the army, perhaps with a cavalry detachment, and to seek out the best place for the night's camp. In the forts along the empire's frontiers other non-combatant men could be found. An entire bureaucracy was necessary to keep the army running; immunes members of the legion were not only the engineers and the technicians, but also the scribes, supervisors, and police in charge of army pay, supplies and customs.

As a unit, a legion was made up of ten cohorts, each of which was further divided into six centuries of eighty men, commanded by a centurion.

The commander of the legion, the legatus, usually held his command for three or four years, often as a preparation for a later term as provincial governor. The legatus, also referred to as general in much of modern literature, was surrounded by a staff of six officers. These were the military tribunes, who - if deemed capable by the legatus - might indeed command an entire section of a legion in battle.

The tribunes, too, were political positions rather than purely military, the tribunus laticlavius being destined for the senate. Another man, who could be deemed part of the general's staff, was the centurio primus pilus. This was the most senior of all the centurions, commanding the first century of the first cohort. In Latin, "primus pilus" means "first javelin". The primus pilus also oversaw the everyday operation of the forces.

Together with non-combatants attached to the army, a legion would count around 6000 men. The 120 horsemen attached to each legion were used as scouts and dispatch riders. They were ranked with staff and other non-combatants and allocated to specific centuries, rather than belonging to a squadron of their own.

The senior professional soldiers in the legion were likely to be the camp prefect, praefectus castrorum. He was usually a man of some thirty years service, and was responsible for organization, training, and equipment.

Centurions, when it came to marching, had one considerable privilege over their men. Whereas the soldiers moved on foot, they rode on horseback. Another significant power they possessed was that of beating their soldiers. For this they would carry a staff, perhaps two or three feet long. Apart from his distinctive armour, this staff was one of the means by which one could recognise a centurion.

Centurions were posted from legion to legion and province to province. It appears they were not only highly sought after men, but the army was willing to transport them over considerable distances to reach a new assignment.

Centurions were not normally discharged but died in service. Thus, to a centurion the army was truly his life. Each centurion had an optio, so called because originally he was nominated by the centurion. The optiones ranked with the standard bearers as principales receiving double the pay of an ordinary soldier. The title optio ad spem ordinis was given to an optio who had been accepted for promotion to the centurionate, but who was waiting for a vacancy.

Another officer in the century was the tesserarius, who was mainly responsible for small sentry pickets and fatigue parties, and so had to receive and pass on the watchword of the day. Finally there was the custos armorum who was in charge of the weapons and equipment.

The first cohort of any legion were its elite troops. So too the sixth cohort consisted of "the finest of the young men", the eighth contained "selected troops", the tenth cohort "good troops". The weakest cohorts were the 2nd, 4th, 7th and the 9th cohorts. It was in the 7th and 9th cohorts one would expect to find recruits in training.

The last major reform of the Imperial Army came under the reign of Diocletian in the late 3rd Century. During the instability that had marked most of that century, the army had fallen in numbers and had lost much of its ability to effectively police and defend the empire. Diocletian quickly recruited a large number of men, increasing the number of legionaries from between 150,000-200,000 to 350,000-400,000, effectively doubling the number, a case of quantity over quality.